1. Field of the Invention
This invention relates to the catalytic hydrogenation of sugars. This invention further relates to the conversion of carbohydrates to lower polyhydric alcohols by a two stage process. More particularly, this invention relates to the production of lower polyhydric alcohols from sorbitol, mannitol, xylitol and the like, by hydrogenolysis in the presence of a sulfide-modified ruthenium catalyst.
2. Description of the Prior Art
Generally, the prior art methods of producing glycerin and short chain polyols from a starting material of reducible sugar and related carbohydrates require at least two separate stages or steps. Usually in the first stage, the starting material, commonly a hexose, is hydrogenated to produce a polyol corresponding the carbon chain length of the starting material. This polyol is referred to as a higher polyhydric alcohol. In the second stage the higher polyhydric alcohol is cracked, that is, a carbon to carbon linkage in the molecule is broken, and hydrogenated further to produce a polyol product of a shorter carbon chain length than the higher polyhydric alcohol fed into the second step. These polyol products of a shorter chain length are called lower polyhydric alcohols. The two stage operation may be carried out in separate reactors or may be accomplished in a single reactor by varying the reaction conditions. Generally, the first stage is carried out under neutral conditions, at a relatively low temperature, at relatively low pressure, and in the presence of a hydrogenation catalyst. The second stage is carried out in the presence of a catalyst, hydrogen and a base such as calcium oxide, at a relatively high temperature and relatively high pressure. The second stage process is a hydrogenolysis. Hydrogenolysis involves the cracking of a carbon to carbon linkage in a molecule with the simultaneous addition of hydrogen to each of the fragments produced by the cracking.
The term "carbohydrate" as used throughout the specification includes monosaccharides and polysaccharides. This term includes both pure compounds, such as glucose, sucrose and cellulose and mixtures such as corn starch hydrolyzate, which is a hydrolysis product of corn starch containing glucose (dextrose) and oligomers thereof or hydrolyzates of cellulose and hemicellulose containing hexoses and pentoses and oligomers thereof.
The term "polysaccharide" as used in the specification includes those saccharides containing more than one monosaccharide unit. This term also encompasses disaccharides and other saccharides containing a small number of monosaccharide units, which are commonly known as oligosaccharides.
The term "higher polyhydric alcohols" as used in the specification and claims refers to those products that are the result of the first stage hydrogenation of carbohydrates. These compounds include sorbitol, mannitol, xylitol, and the like.
The term "lower polyhydric alcohols" as used in the specification and the claims refers to those products that are the result of the second stage hydrogenolysis of higher polyhydric alcohols. These compounds have a maximum of six carbon atoms and a maximum of three hydroxy groups. Lower polyhydric alcohols include ethylene glycol, propylene glycol, glycerol, erythritol, butane diols, and the like.
A wide variety of catalysts have been proposed for the hydrogenation of monosaccharides such as glucose and fructose to polyhydric alcohols. The catalysts most often used for this purpose are Raney nickel catalysts such as those described in U.S. Pat. No. 2,983,734, and finely divided supported nickel catalysts, such as those disclosed in U.S. Pat. No. 2,749,371.
Supported nickel catalysts described in U.S. Pat. Nos. 3,538,019 and 3,670,035 have high activity for the conversion of both monosaccharides and polysaccharides, including carbohydrate mixtures such as corn starch hydrolyzate with high selectivity to sorbitol when either corn starch hydrolyzate or dextrose is used as the starting material. Carbon, diatomaceous earth, and kieselguhr are disclosed as carriers for the catalyst. This represents a significant improvement over U.S. Pat. No. 2,868,847, since relatively inexpensive corn starch hydrolyzate, or other commercially available carbohydrate mixtures, can be used as the starting material in place of much more expensive pure sugars. Various other nickel catalysts for conversion of carbohydrates to polyhydric alcohols are cited in U.S. Pat. Nos. 3,538,019 and 3,670,035.
The conversion of carbohydrates to polyhydric alcohols using ruthenium on a solid carrier is known. U.S. Pat. No. 2,868,847 discloses the use of ruthenium on an inert support, such as carbon, alumina, silica, or kieselguhr, as a catalyst for the catalytic hydrogenation of saccharides such as dextose, levulose, sucrose, maltose, and lactose. Starting materials include monosaccharides, e.g. dextrose and levulose, and disaccharides, e.g. sucrose, lactose, and maltose. Dextrose was hydrogenated to sorbitol. Sucrose and lactose were hydrolyzed and hydrogenated to hexitols. However, maltose, a disaccharide containing two glucose units, was more easily converted to maltitol, a C.sub.12 polyol according to the patent.
U.S. Pat. No. 3,055,840 discloses the hydrogenation of various carbonyl compounds, including glucose (which yields sorbitol on hydrogenation), using a promoted ruthenium catalyst on a solid carrier. Various solid carriers, including carbon, silica gel, alumina, kieselguhr, and titanium dioxide, are disclosed.
U.S. Pat. No. 3,963,788 describes and claims a process for the conversion of a carbohydrate to a polyhydric alcohol using a ruthenium-containing zeolite having a silica/alumina mole ratio greater than 3, particularly a ruthenium-containing Y type zeolite, as the catalyst. The ruthenium is present as the free metal on the zeolite which serves as a support. Glucose and corn starch hydrolyzate, both of which yield sorbitol are representative carbohydrates. U.S. Pat. No. 3,963,789 describes and claims a process for conversion of a polysaccharide-containing carbohydrate, such as corn starch hydrolyzate, to a polyhydric alcohol using ruthenium on a crystalline aluminosilicate clay as the catalyst. Both of these applications describe regeneration of the catalyst with a dilute aqueous mineral acid.
U.S. Pat. No. 4,072,628 teaches the regeneration of a supported ruthenium catalyst used for the conversion of carbohydrates to polyhydric alcohols. This is a first stage hydrogenation. The catalyst is contacted with an aqueous solution of a mineral acid, such as sulfuric, hydrochloric or phosphoric acid.
The above-cited references describe a variety of catalytic processes for the conversion of carbohydrates to polyhydric alcohols, however, none describe the use of a sulfide-modified ruthenium catalyst in the second stage hydrogenolysis of polyhydric alcohols, such as sorbitol, to lower polyhydric alcohols. The use of a sulfide-modified catalyst in this second stage allows a greater range of reaction temperatures to be used and a higher product selectivity. The traditional hydrogenation catalyst generally result in a wide range of products. Typical reaction products include methane, methanol, ethanol, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, glycerine, erythritol, xylitol, and the like. Unmodified ruthenium generally produces a large quantity of methane. By contrast, using a sulfide-modified ruthenium catalyst and a base, these large amounts of methane can be eliminated. This provides greater selectivity. The sulfide-modified ruthenium catalyst results in a product mixture comprised largely of ethylene glycol and 1,2-propylene glycol. Under the appropriate reaction conditions, approximately 91% of the product mixture may be composed of these two glycols. These glycols have application in a wide variety of products. Typical uses include polyester fiber intermediates, polyurethane intermediates, plasticizer intermediates and detergents.